The present invention relates to a crystal puller for growing single crystal semiconductor material, and more particularly to a heat shield assembly adapted to be incorporated in a crystal puller.
Single crystal semiconductor material, which is the starting material for fabricating many electronic components, is commonly prepared using the Czochralski (xe2x80x9cCzxe2x80x9d) method. In this method, polycrystalline semiconductor source material such as polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is melted in a crucible. Then a seed crystal is lowered into the molten material (often referred to as the melt) and slowly raised to grow a single crystal ingot. As the ingot is grown, an upper end cone is formed by decreasing the pull rate and/or the melt temperature, thereby enlarging the ingot diameter, until a target diameter is reached. Once the target diameter is reached, the cylindrical main body of the ingot is formed by controlling the pull rate and the melt temperature to compensate for the decreasing melt level. Near the end of the growth process but before the crucible becomes empty, the ingot diameter is reduced to form a lower end cone which is separated from the melt to produce a finished ingot of semiconductor material.
To increase throughput of the crystal puller, it is desirable to increase the pull rate xe2x80x9cvxe2x80x9d at which the crystal is pulled up from the melt. However, simply increasing the pull rate, by itself, can be detrimental to the growth and quality of the crystal. For example, an increase in pull rate can result in distortion of the ingot diameter if the ingot is not given sufficient time to cool and solidify as it is pulled up from the melt.
Also, some wafer quality characteristics, such as Gate Oxide Integrity (GOI), are effected by a change in pull rate. Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as metal oxide semiconductor (MOS) devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by the agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The GOI of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
One way to improve GOI is to control the number of vacancies grown into the ingot upon solidification of the ingot as it is pulled up from the melt. It is understood that the type and initial concentration of vacancies and self-interstitials, which become fixed in the ingot as the ingot solidifies, are controlled by the ratio of the growth velocity (i.e., the pull rate v) to the local axial temperature gradient in the ingot at the time of solidification (Go). When the value of this ratio (v/Go) exceeds a critical value, the concentration of vacancies increases. Thus, to inhibit an increase in the concentration of vacancies, i.e., to avoid increasing the ratio v/Go, the axial temperature gradient at the solid-liquid interface must be correspondingly increased if the pull rate v is increased.
It is well known to increase axial temperature gradient at the solid-liquid interface by positioning a radiation screen or heat shield assembly above the melt surface between the crucible side wall and the growing ingot for shielding the ingot from the heat of the crucible side wall. For example, co-assigned U.S. Pat. No. 6,197,111, which is incorporated herein by reference, discloses a heat shield assembly 50 including insulation 52 contained between coaxially positioned inner and outer reflectors 44, 46. Heat from the crucible 20 is transferred to the outer reflector and to the insulation. Heat transfer from the outer reflector to the inner reflector is inhibited by the insulation and by the minimal direct contact between the inner reflector and the outer reflector. Thus, as the ingot is pulled from the melt, heat is transferred from the ingot to the inner reflector more rapidly. Accordingly, the heat shield assembly increases the axial temperature gradient of the ingot as it is pulled up from the melt, which allows the pull rate of the crystal puller to be increased without distorting the growing ingot.
The performance of the shield assembly of the ""111 patent is generally satisfactory. However, the assembly is very expensive to manufacture and maintain, especially when it is sized for large diameter crystal pullers, because each reflector is made of a large, isomolded graphite billet. Generally, larger billets of such material are very expensive, which drives up the cost of the reflector. Moreover, maintenance of the reflectors is expensive because they must be occasionally replaced due to damage. The reflectors must be near the melt for optimum functionality, but the proximity to the melt exposes the reflectors to damage. The reflectors are typically damaged by silicon splatter from the melt or by inadvertent dipping into the melt. Additionally, the reflectors are subject to substantial thermal gradients, due at least in part to the insulation between the reflectors, especially along the bottom of the shield assembly. The gradients typically cause stress and consequent radial cracks in the outer reflector, and such cracks occur more often in larger reflectors. If the damaged reflector is not immediately replaced, particles from the reflector or the insulation therein may be introduced to the melt and thereby cause defects (e.g., LZDs) in the ingot.
Also of interest is U.S. Pat. No. 5,824,152, which shows a radiation screen 1 designed to increase the time the xe2x80x9cbottom portionxe2x80x9d of the ingot, i.e., the portion at the solid-liquid interface, spends in a temperature region between 1000xc2x0 and 1200xc2x0 C. Such a screen design is different from conventional screens, which screen the bottom portion of the ingot from radiant heat so that the ingot passes through the temperature region quickly. (Col. 3, line 66). As shown in FIG. 1 of the patent, the radiation screen 1 is divided into an upper screen 2 and a lower screen 3. Upper screen 2 is of three-layer construction comprising outer graphite members 2b and 2c enclosing heat-insulating member 2a. The lower screen 3 includes a flange for engaging a ledge portion at the bottom end of upper screen 2. The lower screen 3 appears to extend less than about 30% of the total height of the screen 1, and it is of single-layer construction so that the lower screen provides little insulation at the solid-liquid interface. Accordingly, the radiation screen is not designed to substantially increase the axial temperature gradient at the solid-liquid interface.
Among the several objects and features of the present invention may be noted the provision of a heat shield assembly for a crystal puller that is relatively inexpensive to manufacture and maintain; the provision of such a heat shield assembly that permits modular replacement of damaged parts; the provision of such a heat shield assembly that is suitable for large diameter ingots; the provision of such a heat shield assembly that is damage resistant; and the provision of such a heat shield assembly capable of substantially increasing the axial temperature gradient of the solid-liquid interface to allow the pull rate of the crystal puller to be increased.
Briefly, a heat shield assembly of the present invention is used in a crystal puller for growing a monocrystalline ingot from molten semiconductor source material. The crystal puller has a housing, a crucible contained in the housing for holding molten semiconductor source material and a heater in thermal communication with the crucible for heating the crucible to a temperature sufficient to melt the semiconductor source material held by the crucible. A pulling mechanism is positioned above the crucible for pulling the ingot from the molten source material held by the crucible and a support structure is positioned above the molten source material for supporting the heat shield assembly above the molten source material. The heat shield assembly has a central opening sized and shaped for surrounding the ingot as the ingot is pulled from the molten source material. The heat shield assembly comprises an outer shield adapted for positioning between the ingot and the crucible as the ingot is pulled from the molten source material. The outer shield includes a first section and a second section, the second section extending generally downward from the first section toward the molten material. The assembly further comprises an inner shield adapted for positioning generally between the ingot and the outer shield. The inner shield also includes a first section and a second section, the second section extending generally downward from the first section toward the molten material. The second section of at least one of the inner and outer shields is releasably connected to the respective first section of the same shield so that, in the event one of said releasably connected first and second sections is damaged, the damaged section may be separated from the other section of the shield to allow replacement with an undamaged section without replacement of the other section of the shield.
In another aspect of the invention, the heat shield assembly comprises an upper section interposed between the ingot and the crucible as the ingot is pulled from the molten source material and a lower section extending generally downward from the upper section toward the molten material. The lower section has a height equal to at least about 33% of a height of the heat shield assembly. The lower section has a top end releasably connected to a bottom end of the upper section so that, in the event one of said upper and lower sections is damaged, the damaged section may be separated from the other section of the shield to allow replacement with an undamaged section without replacement of the other section of the shield.
In yet another aspect of the invention, the heat shield assembly comprises an outer shield and an inner shield. A bottom of the shield assembly is defined at least in part by a bottom end of one of the inner and outer shields. A ring is disposed between the outer and inner shields for reducing the thermal gradient across the bottom end to inhibit cracking in said one of the outer and inner shields.
In still another aspect of the invention, the heat shield assembly comprises an outer shield, an inner shield, an annular chamber defined between the inner shield and outer shield, and insulation having a lower end disposed in the chamber. A bottom end of at least one of the outer and inner shields engaging the lower end of the insulation so that the bottom end is subject to a thermal gradient due to the insulation. The bottom end extending only partway across the lower end of the insulation to reduce the thermal gradient in said at least one of the outer and inner shields and thereby inhibit cracking of the shield.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.